The present disclosure relates to the preparation of the rendering with a color output device of an object or several objects described in a page description language.
Portable Document Format (also known as PDF) is a file format used to describe documents with a page description language. It is more and more adopted in the graphics and printing industry as a data interchange format. A PDF file encapsulates a complete description of a fixed-layout flat document, including text, images and some information to render it. A rendering engine is used to read the description and convert it to pixels forming an image that can be output on an output device that can be a display device, a printer device or another kind of output device.
The PDF format support many features, including colors, overprinting objects and transparencies.
Colors are often defined in the PDF documents in the CMYK color model that is a subtractive color model referring to the four inks used: cyan, magenta, yellow, and black. The CMYK model is widely used in the offset printing industry (and in printing systems in general), and this has had a large role on the widespread usage of the CMYK color model in PDF files.
The CMYK model used in the description of PDF files is either untagged, leaving undefined the parameters defining the precise color profile to use, which will be the one of the output device, or tagged, which means that parameters defining a precise color profile used to describe the document are specified in the document.
Overprinting objects is obtained by blending the objects to generate the pixels to print, whereas transparency is a further feature that allows objects to interact with each other to produce blending effects. A color blending operation is defined, inter alia, by a blend color space that is used to compute the color of pixels resulting from the blending of the objects. The blend color space is usually a CMYK color space, because the CMYK model is well adapted to perform blending.
The output device may however not operate in a CMYK space. For example, many printers and visual display screens use the RGB color model. The RGB acronym refers to Red, Green and Blue and this model is an additive color model. Like CMYK, RGB is a device dependent model.
The final ink space is not necessarily RGB, but in that case, RGB is used as an intermediate space. There are printers operating with 6, 7-8 inks or even with 12 inks. In such cases, and in order to hide the complexity of multilink separations, RGB is a suitable pick as device color model.
When the objects are defined in a CMYK color space in the description language or when a CMYK blending space is used, a conversion from the CMYK space to the RGB space has to be applied. Known conversions include calibrated and uncalibrated conversions.
Many features and advantages of the above-described method will become readily apparent from the following detailed description and accompanying drawings.